Note that this is an unusual yesod-auth-* package. We do not
define an AuthPlugin because the user can't log in to your
website via Desk.com (it's the opposite, they login in Desk.com
via your website). Instead, this plugin defines an Yesod
subsite that you may include on your site; the subsite defines
a route that accepts incoming users from Desk.com and
authenticates them.